Different applications require different power and voltage levels and therefore different types of converters that have different power ratings. In general power converters can be divided into three different types, low voltage, medium voltage and high voltage where each of the different converter types includes a rectifier section that converts three phase AC voltage to DC voltage on positive and negative DC buses and across a DC link capacitor and an inverter section that converts the DC voltage on the DC buses to three phase AC voltage on three output lines that feed a three phase load. The difference between the three converter types is typically in the types of devices employed to configure the converters where the power rating and expense associated with components increases as the power rating of the drive increases. Thus, high voltage converters are typically more expensive than medium voltage converters and low voltage converters are less expensive than medium voltage converters.
There are a huge number of different applications that require low voltage converters and fewer applications that require medium and large voltage converters and, for this reason, low voltage converters are manufactured in much greater quantities than the other converter types and economies of scale are achieved such that low voltage converters are relatively inexpensive. In fact, low voltage converter costs are so much less expensive relative to medium and high voltage converters that, in many cases, it is less expensive to cobble together several low voltage converters to serve medium and or high voltage applications.
One simple way to connect low voltage converters to provide a medium or high power rating is to connect a plurality of low voltage converters to form a cascaded H bridge multi-level inverter system where each low voltage single phase H bridge cell is isolated from input power lines using a multiphase transformer and where each single phase of the configuration is operated as a single phase. While this solution has several advantages, this solution also has several shortcomings. First, because each H bridge in this configuration type operates as a single phase, the size of the DC link capacitors required to configure the bridges is much higher than for a three phase converter which increases costs appreciably. Second, each of the three cells has to be controlled simultaneously to balance the power generated by each cell in order to guarantee safe drive operation and to create a high quality input current waveform that meets IEEE 519 requirements related to harmonics. Third, in cases where regeneration is to be supported, many more controlled switches have to be added to the overall system which increased the price of the overall system substantially.
One other solution that has been considered in the conversion industry has been to operate low voltage three phase converters that include input lines that are connected via non-isolated transformers and output lines that are connected via non-isolated transformers. Here, one problem has been that the non-isolated transformers linked to the output lines leave the low voltage drives exposed to high motor terminal voltages that can cause damage to the low voltage drives when certain operational conditions occur.